economic impact of derated climb on large commercial engines

8/6/2019 Economic Impact of Derated Climb on Large Commercial Engines

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2007 Performance and Flight Operations Engineering Conference


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Rick Donaldson, Dan Fischer, John Gough, Mike Rysz | Economic Impact of Derated Climb on Large Commercial Engines

8.1

Introduction

Aircrat engines are sized and power managed to meet takeo eld length and climbrate requirements at the maximum takeo gross weight (TOGW). When operatingat reduced TOGW, reduced thrust (derate) may be used in both takeo and climbto extend engine lie and reduce maintenance cost. The benet o takeo derate is

clear because takeo is typically the most severe operating condition or fowpathcomponents and its impact on mission time and uel is minimal because o its shortduration. Consequently, most operators make requent and systematic use o takeoderate.

The net impact o using climb derate is less obvious because it involves opposinginfuences whose magnitudes depend on the particular aircrat and engine inquestion. Use o climb derate increases the time and distance to climb to cruisealtitude. This will generally result in a small increase in block uel burn because asmaller raction o the total range will be fown at the most ecient cruise altitudeand power setting. It may however result in a small reduction in maintenance cost.The existence and magnitude o the maintenance cost reduction depend on theseverity o climb operation relative to takeo and the specic ailure modes thatdrive engine lie. Because our-engine aircrat typically require a higher raction otakeo thrust in climb than twin-engine aircrat do, climb operation is more likely toinfuence engine lie on our-engine aircrat. However i the ailure modes that drivethe engine o wing are predominately related to low cycle atigue, engine lie willonly be aected by takeo thrust as long as it is not reduced below the climb rating.Other ailure modes such as oxidation and corrosion are more likely to be infuenced

by climb thrust rating because they are dependent on time at temperature, butreducing climb thrust is not always benecial because it increases time o exposure

while it reduces temperature. In addition, reducing climb thrust may move materialtemperatures rom the oxidation regime to the corrosion regime, which may be themore lie limiting ailure mode.

The purpose o this paper is to examine these conficting infuences in order to provideoperators guidance as to the use and potential benet o using climb derate.

Approach

In order to gain an understanding o the economic impact o climb derate several wide body aircrat were examined, including both twin-engine and our-enginecongurations. Mission analyses were perormed at maximum and derated climbthrust with maximum passenger loads at 3,000 nm ranges or long-range applicationsand 800 nm or short-range applications. The passenger loads and ranges wereselected to produce TOGWs consistent with operation at takeo thrust deratelevels typically used in service.

Time, temperature and speed histories rom the mission analyses were usedto evaluate the eect o climb derate on the lives o the critical parts known todominate the time on wing o each engine type examined. The mission uel burnand lie impacts derived rom these analyses were then combined to assess the netoperational cost impact as a unction o climb derate.

Results

O the congurations examined, the GEnx-1B54 powered 787-3 and GEnx-1B70


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8.2

powered 787-9 provide the best illustrative examples. While both o these aircratare twin-engine wide bodies, they require signicantly dierent levels o takeothrust rom the same engine model and consequently have very dierent climbseverities relative to takeo. They are also intended or very dierent purposes, the787-3 being a short haul aircrat and the 787-9 being a long haul aircrat. Thesedierences drive dierent responses to the use o climb derate, the study o which is

useul in extrapolating the results to other aircrat.

The 787-3 and 787-9 results will be presented in some detail in the sections thatollow. Some observations as to how the results rom other aircrat diered romthese examples will also be oered.

Mission Analysis and Fuel BurnLike earlier Boeing aircrat, 787 pilots have two basic climb derate selections: CLB1corresponding to a nominal 10% derate and CLB2 corresponding to a nominal 20%derate. The actual climb thrust reduction corresponding to the two derate levels ispresented as a unction o altitude in Figure 1. The nominal reduction is eectiveup to 10,000 t, at which point it is aired out to zero. The deault rate at which itairs out is an option pre-selected by the airline. The ast taper (FT) option airs outthe nominal derate between 10,000 t and 15,000 t. The slow taper (ST) optionairs out the nominal derate between 10,000 t and 30,000 t. Unlike earlier models,the 787 will give the pilot additional fexibility in selecting both derate level and thealtitude at which the derate airs out to zero. As will be shown later, the derate leveland air-out altitude can signicantly aect economic perormance.

Reducing climb thrust through the use o derate reduces climb rate and henceincreases the time required to reach cruise altitude, as shown in Figure 2. At thereduced TOGWs studied, these time increases are relatively small, none beinggreater than 2.5 minutes. Moreover, climb time is well under the 30-minute airtrac control maximum in all cases.

Figure 1. Climb thrustderate options

Thrust

reduction

0%

5%

10%

15%

20%

25%

0 10,000 20,000 30,000 40,000

Altitude (ft)

CLB1-FT

CLB2-FT

CLB1-ST

CLB2-ST


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8.3

Similarly, use o climb derate increases the distance to climb by a small amount. Asshown in Figure 3, the maximum increase is 13 nm and cruise altitude is reached

well beore the 200 nm air trac control limit. Increased time and distance to cruisealtitude should thereore not be an impediment to the use o climb derate.

Figure 2. Impact o climb

derate on time to cruisealtitude

Figure 3. Impact o climbderate on distance to cruisealtitude

Time,

minutes

0

5

10

15

20

25

Max climb CLB1-FT CLB1-ST CLB2-FT CLB2-ST

Climb thrust reduction/taper

Climb time to 39000 ft cruise

787-3/GEnx-1B54

787-9/GEnx-1B70

Distance,

nm

0

50

100

150

Max climb CLB1-FT CLB1-ST CLB2-FT CLB2-ST

Climb thrust reduction/taper

Climb distance to 39000 ft cruise

787-3/GEnx-1B54

787-9/GEnx-1B70


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8.4

As shown in Figure 4, increased distance to cruise altitude does result in a small block uel burn increase because as more range is traveled in climb, less range istraveled at the more ecient cruise altitude and power setting.

In terms o cost, this penalty can be as much as $1 per EFH or the 787-9 and $7per EFH or the 787-3, as shown in Figure 5. The penalty per EFH is larger or the787-3 because the shorter range magnies the impact o climb, i.e. there are moreclimb segments fown per EFH. In both applications this penalty will tend to osetpotential maintenance cost benets, which will be examined next.

Climb vs Takeoff SeverityThe part lie consumed during any portion o the fight is determined by thetemperature and stress the part experiences. For some ailure modes, such as

Figure 5. Fuel cost penaltyor reduced thrust climb

Figure 4. Block uel burnpenalty or climb derate

Block fuel

penalty,

USG/trip

0

5

10

15

CLB1-FT CLB1-ST CLB2-FT CLB2-ST

Climb derate/taper

787-3/GEnx-1B54

787-9/GEnx-1B70

Fuel cost

penalty,

dollars/EFH

0

5

10

CLB1-FT CLB1-ST CLB2-FT CLB2-ST

Climb derate/taper

787-3/GEnx-1B54

787-9/GEnx-1B70


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8.5

oxidation, time o exposure is also a actor. The individual part temperaturesand stresses are closely related to two key gas path temperatures: high-pressurecompressor (HPC) discharge temperature and high-pressure turbine (HPT) inlettemperature. The impact o takeo and climb derate on HPC discharge temperatureis presented in Figure 6 or the GEnx-1B54 and Figure 7 or the GEnx-1B70. Similartrends in HPT inlet temperature are presented in Figure 8 and Figure 9. From these

gures it is readily apparent that maximum climb temperatures become more severethan takeo temperatures once takeo thrust is reduced by approximately 7% orthe GEnx-1B54 and 17% or the GEnx-1B70. The separation between maximumtakeo and maximum climb severity is larger or the GEnx-1B70 because it is nearthe top end o GEnx takeo thrust ratings.

Figure 6. 787-3 / GEnx-1B54 HPC dischargetemperature severity

Figure 7. 787-9 / GEnx-1B70 HPC dischargetemperature severity

0 5 10 15 20 25

24 deg F

Time from brake release (min)

HPCdischarge

temperature

Max climb

CLB1-FT

CLB2-FT

CLB1-ST

CLB2-ST

Max takeoff

90% takeoff

80% takeoff

70% takeoff

60% takeoff

0 5 10 15 20 25

61 deg F


HPC

discharge

temperature

Max climb

CLB1-FT

CLB2-FT

CLB1-ST

CLB2-ST

Max takeoff

90% takeoff

80% takeoff70% takeoff

60% takeoff


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8.6

It should also be noted that although the temperature severity is signicantly greaterat maximum takeo, the time o exposure is much greater in climb. Failure modessuch as oxidation, corrosion and creep rupture will thereore be heavily dependenton climb severity and climb time.

Engine Life ImpactIn order to assess engine lie impact, the specic engine removal drivers or eachengine model were evaluated to identiy the critical parts and ailure modes thatdetermine time on wing. These critical parts and ailure modes were urtherexamined to identiy the subset that is sensitive to takeo and climb thrust levels.Lie analyses were then conducted on this subset at various levels o takeo andclimb thrust to quantiy each individual part lie impact.

0 5 10 15 20 25

60 deg F


HPT

inlet

temperature

Max climb

CLB1-FT

CLB2-FT

CLB1-ST

CLB2-ST

Max takeoff

90% takeoff

80% takeoff

70% takeoff

60% takeoff

0 5 10 15 20 25


HPT

inlet

temperature

181 deg F

Max climb

CLB1-FT

CLB2-FT

CLB1-ST

CLB2-ST

Max takeoff

90% takeoff

80% takeoff

70% takeoff

60% takeoff

Figure 8. 787-3 /GEnx-1B54 HPT inlettemperature severity

Figure 9. 787-9 /GEnx-1B70 HPT inlettemperature severity


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8.7

In the case o the GEnx, two critical ailure modes were analyzed: combustorcorrosion and high-pressure turbine (HPT) stage 1 blade oxidation. Combustorcorrosion results are presented in Figure 10 or the GEnx-1B54 and Figure 11 orthe GEnx-1B70. Because corrosion increases with exposure time in the corrosiontemperature regime, reducing climb thrust actually reduces corrosion lie slightly.This eect is more pronounced on the GEnx-1B70 because the high temperatures

associated with the increased thrust rating ensure that the combustor operates wellinto the corrosion regime. The cooler GEnx-1B54 actually exhibits an improvementin corrosion lie between 10% and 20% climb thrust reduction owing to parts o theclimb segment moving to a less severe portion o the corrosion regime. For bothratings, the ast taper produces somewhat better lie because it reduces time to climband hence reduces the time in the corrosion regime.

Figure 10. 787-3 / GEnx-1B54 combustor corrosionlie

Figure 11. 787-9 / GEnx-1B70 combustor corrosionlie

Climb thrust reduction

80%

90%

100%

110%

120%

0% 10% 20%

Relative

life

Max TO / FT

Max TO / ST

-20% TO / FT

-20% TO / ST

80%

90%

100%

110%

120%

0% 10% 20%

Climb Thrust Reduction

Max TO / FT

Max TO / ST

-20% TO / FT

-20% TO / ST

Relative

life


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8.8

HPT stage 1 blade oxidation lie analysis results are presented in Figure 12 or theGEnx-1B54 and Figure 13 or the GEnx-1B70. Like corrosion, oxidation damageincreases with increasing temperature and time o exposure. However temperatureis by ar the dominant actor so that HPT stage 1 blade oxidation lie improvessignicantly with climb thrust reduction despite the increased time to climb. Despitethe increased time to climb, slow taper produces the most improvement because it

reduces temperatures over the entire climb segment.

The results o these individual part ailure mode analyses were then combined todetermine the net sensitivity o part lie to takeo and climb thrust reduction. Inthis combination process, the most limiting ailure mode (i.e. the one in which thepart has the lowest lie) will dominate the part lie sensitivity, but other ailure modesthat are insensitive to thrust reduction will tend to reduce the overall sensitivity.

Finally, the results o the individual part lie sensitivities were combined to assess

Figure 12. 787-3 / GEnx-1B54 HPT Stage 1 bladeoxidation lie

Figure 13. 787-9 / GEnx-1B70 HPT Stage 1 blade

oxidation lie

0% 10% 20%


Max TO / FT

Max TO / ST

-20% TO / FT

-20% TO / ST

80%

100%

120%

140%

160%

180%

200%

220%

Relative

life

0% 10% 20%


Max TO / FT

Max TO / ST

-2 0% TO / FT

-2 0% TO / ST

80%

100%

120%

140%

160%

180%

200%

220%

Relative

life


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8.9

the overall engine lie impact. Because engine lie is determined by multiple parts,some o which are insensitive to takeo and climb thrust levels, overall engine lietends to be less sensitive to climb thrust variation than the individual critical partsdiscussed in the preceding paragraphs. The resulting engine time on wing sensitivityis presented in Figure 14 or the GEnx-1B54 and Figure 15 or the GEnx-1B70.

These results illustrate the interactions between various ailure modes whenindividual part lives are rolled up to produce a time on wing. Note that the slowtaper produces superior engine time on wing or the GEnx-1B54, indicating thatthe HPT stage 1 blade lie is the dominant eect. In the case o the GEnx-1B70,

Figure 14. 787-3 / GEnx-1B54 time on Wing

Figure 15. 787-9 / GEnx-1B70 time on wing

90%

100%

110%

120%

0% 10% 20%


FT

ST

Relative

time on

wing

787-9 / GEnx-1B54 time on wing@ 24% takeoff thrust reduction

90%

100%

110%

120%

0% 10% 20%


FTST

Relative

time on

wing

787-9 / GEnx-1B70 time on wing

@ 19% takeoff thrust reduction


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8.10

the negative impact o slow taper on combustor corrosion is somewhat stronger andas a result time on wing is slightly less than with the ast taper. These interactions

between ailure modes drive dierences among various engine models maintenancecost sensitivity to climb thrust reduction.

The overall engine lie sensitivities or each engine model were then applied to the

base maintenance costs to obtain maintenance cost per hour contributions to theoverall operating cost sensitivity.

Operating Cost ImpactThe net operating cost impact o using reduced climb thrust was evaluated by addingthe uel burn and maintenance cost sensitivities described in the preceding sections.The results are presented as a unction o climb thrust reduction at typical reducedtakeo thrust levels in Figure 16 or the 787-3 and Figure 17 or the 787-9. For boththe 787-3 and 787-9, the uel burn penalty associated with reduced climb thrustopposes the maintenance cost benet such that the net operating cost is reducedsomewhat between 0 and 10% climb thrust reduction. Beyond 10% climb thrustreduction the maintenance cost benet levels o and the uel burn penalty becomessteeper, resulting in a net operating cost increase.

Figure 16. 787-3 / GEnx-1B54 Operating CostImpact

$(10)

$(5)

$

$5

$10

0% 10% 20%


787-3 / GEnx-1B54 operating cost impact

@ 24% takeoff thrust reduction

FT Maintenance Cost

ST Maintenance Cost

FT Fuel Cost

ST Fuel Cost

FT Total Cost

ST Total Cost

Operating

cost

impact($/EFH)


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8.11

Figure 17. 787-9 / GEnx-1B70 Operating CostImpact

While the general trends are similar or both 787 models, the magnitude o thepotential benet is somewhat dierent. The power management schedules or the787-9 model are typical o twin-engine aircrat in that takeo temperatures aremuch higher than climb temperatures. As a result, the net benet o reduced thrustclimb is small, $1.80 per EFH with ast taper and $1.40 with slow taper. Because theGEnx-1B54 is signicantly derated rom the maximum capability, there is a muchsmaller dierence between takeo and climb temperatures. Consequently, the net

benet o reduced thrust climb is larger, $3.40 per EFH with ast taper and $4.70per EFH with slow taper.

It should be noted that the results presented above assume a typical level o takeothrust reduction, 24% or the GEnx-1B54 and 19% or the GEnx-1B70. When higherlevels o takeo thrust are used, the maintenance cost benet o reduced thrust climb

will be less because a smaller portion o the engine lie is consumed in climb. Whenhigher levels o takeo thrust are required due to high TOGW, the maximum climbrate will be lower and the uel burn penalty or reduced thrust will be higher. Thesetwo eects combine to make reduced thrust climb increasingly less attractive.

Similarity to Other AircraftDespite the dierences in engines, ratings and engine lie drivers, the other aircratstudied showed remarkably similar results: in most cases, use o 5% to 10% climbthrust reduction produces a small net economic benet on fights where signicantreduced thrust takeo (15% to 20% or greater) is used. The magnitude o themaintenance cost benet tends to be greater or low takeo thrust ratings and or

our-engine aircrat because climb consumes a relatively larger portion o the enginelie. The magnitude o the opposing uel burn penalty tends to increase as the timeto climb at maximum rated climb increases. Hence as the inherent climb rate o theaircrat decreases, the likelihood o realizing a net benet rom the use o reducedthrust climb decreases.

$(10)

$(5)

$

$5

$10

0% 10% 20%


Operatingcostimpact($/EFH)

787-3 / GEnx-1B70 operating cost impact@ 19% takeoff thrust reduction

FT Maintenance Cost

ST Maintenance Cost

FT Fuel Cost

ST Fuel Cost

FT Total Cost

ST Total Cost


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Conclusions & Recommendations

This paper is a result o GE/CFM customer inquiries into uel and maintenancecost savings and other industry papers recently published on this subject. Based onthe results presented as well as similar results or other GE-powered aircrat, smallreductions in climb thrust can produce small reductions in net operating cost due

to a avorable trade between maintenance cost and uel cost. For short haul aircratrequiring lower thrust ratings, the best uel savings prole would be using as muchreduced takeo thrust as possible ollowed by the application o normal climbpower. This can yield a uel savings o between $3-$7 dollars per fight hour over asimilar prole that uses derated climb. Consideration must be given to the increasein engine operating cost as described previously. For long range aircrat there is nosignicant uel burn or maintenance cost saving in using normal climb over a deratedclimb. Maintenance cost savings tend to reach a maximum somewhere between 5%and 10% climb thrust reduction, ater which the increase in uel burn dominates thetrend and the two end up osetting each other. This result is applicable to fights on

which TOGW is low enough to permit typical levels o takeo thrust reduction in the15% to 20% range. When higher levels o takeo thrust are required, the optimum

climb thrust will be higher and the potential benet o climb thrust reduction willdiminish and eventually disappear.

Consequently, it is recommended that use o reduced thrust climb be restricted tofights where signicant reduced thrust takeo, 15% or greater, is used or maximumsavings on short haul aircrat. For long-range aircrat there is no signicant impacton uel or maintenance cost savings, regardless o the amount o reduced thrustclimb.

In those cases where reduced thrust climb is used, the climb thrust reduction shouldnot exceed 10%. Furthermore, climb thrust should not be reduced to the extentthat climb time will exceed 25 minutes, as this is likely to produce an unacceptableincrease in uel cost. Many fight management systems automatically select a level

o derate or climb dependent on the level o derate used or takeo. Any changeto procedures, per the aorementioned recommendations, should be coordinated

between the airline fight operations departments and the aircrat manuacturer.

economic impact of derated climb on large commercial engines

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